This invention relates generally to power supplies, and, more particularly, to power supplies that use push-pull, semi-bridging self-oscillating circuits in providing low voltages at high frequencies to loads such as gas discharge lamps.
Power supplies for lamps commonly provide some 110 volts from a line source or ballast. Flourescent lamps, for example, employ such at high frequency to avoid visible flickering. However, discharge lamps, such as halogen lamps, require only some 12 volts of potential. Being a gas discharge device, they cannot operate on DC current. Thus they have a somewhat unique power supply requirement of high frequency yet low voltage with the resulting benefit of low power requirement.
Halogen lamps are usually powered by push-pull, semi-bridged self oscillating circuits. An example of such circuit comprises a pair of switching devices arranged in series between the output terminals of a rectifier. The rectifier receives power from an AC power source via a filter circuit and converts the AC voltage to pulsating DC voltage. The switching devices are connected in parallel with two series capacitors. Two diodes for supplying currents in opposite directions to the load are connected in parallel with the switching devices and with the output terminals of the rectifier. The node of the series diodes is connected with the node of the series switching devices. The capacitance of one capacitor (xe2x80x9cLarger Capacitorxe2x80x9d) is greater than that of the other capacitor (xe2x80x9cSmaller Capacitorxe2x80x9d). A diode is connected in parallel with the Smaller Capacitor. A series circuit of an inductor and a discharge lamp is connected between the node of the switching devices and the node of the series capacitors. A third capacitor (xe2x80x9cLoad Capacitorxe2x80x9d) is connected between the end terminals of the discharge lamp. The discharge lamp and Load Capacitor therefore become part of two series loops. In one loop, the discharge lamp/Load capacitor is in series with the Smaller Capacitor, a switching device (xe2x80x9cFirst Switching Devicexe2x80x9d) and the inductor. In the other loop, the discharge lamp/Load Capacitor is in series with the inductor, the other switching device (xe2x80x9cSecond Switching Devicexe2x80x9d) and the Larger Capacitor.
The switching devices are controlled by a switching control circuit that is also powered via the rectifier. The switching control circuit causes the switching devices to switch On and Off in an alternating manner at a very high frequency. In the ON period of one switching device, current is supplied from the rectifier to the inductor, the discharge lamp, and the Larger Capacitor via the switching device. During this ON period, the Larger Capacitor is charged. In the time interval after the switching device is turned off, and before the other switching device is turned on, the inductor supplies current to the discharge lamp and the Larger Capacitor. When the other switching device is turned on, the Larger Capacitor discharges via the discharge lamp, the inductor and the other switching device. In the time interval after the other switching device is turned off, and before the one switching device is turned back on, the inductor supplies current to the discharge lamp and the Smaller Capacitor via one of the in-series diodes.
A problem with such a push-pull oscillating circuit is that during the period when the pulsating output voltage from the rectifier is at or near zero, no input current is momentarily supplied to the switching control circuit because the voltage supply is below the breakdown voltage of one or more components of the switching control circuit. This results in a discontinuous current waveform being supplied to the gas discharge lamp thereby limiting the power factor of the power supply. Thus if a gas discharge lamp power supply could be devised having the high frequency discharge of a push-pull, semi-bridging oscillating circuit, yet with a power factor greater than 99%, a distinct advantage could be achieved. Accordingly, it is the provision of such that the present invention is primarily directed.
A power supply for a gas discharge lamp comprises an oscillating circuit having two switches connected in series at a node and controlled by an oscillation ignition circuit that has a diode that operates above a selected voltage threshold level. An energy storage circuit is connected to the node of the oscillating circuit switches and to the oscillation ignition circuit for supplying energy to the oscillation ignition circuit and the oscillating circuit while output voltage is below the selected voltage threshold level of the diode. It has been found that this circuit serves to provide a power supply power factor in excess of 99%.